The Russo Brothers were recently quoted as saying that Steve Rogers lived his life out in an alternate past before returning to the original timeline to pass the shield and mantle of Captain America on to Sam Wilson. Markus, however, doesn't seem to agree with the "alternate reality" theory as he dubs it.
"That is our theory," he says when asked if there were two Captain Americas in existence for a time. "We are not experts on time travel, but the Ancient One specifically states that when you take an Infinity Stone out of a timeline it creates a new timeline. So Steve going back and just being there would not create a new timeline. So I reject the "Steve is in an alternate reality" theory."
"I do believe that there is simply a period in world history from about '48 to now where there are two Steve Rogers," he continues, further confusing matters. "And anyway, for a large chunk of that one of them is frozen in ice. So it's not like they'd be running into each other."